New flexible air duct

I have an older home that doesn't have any duct work from the air return vents to the furnance. The air gets pulled from the crawl space under the house and into the furnance. It's an odd setup. There are 4 air return vents in the house, just no duct work behind them. So I plan on installing the flexible duct in the crawl space. The flexible ducts are not my concern. My question is:
What goes on the end of the duct work. The air returns are about 12 inches wide and 8 inches high and the opening into the furnance is about the size of a standard air filter. Will these 'transitions' need to be custom made or should I be able to find them at a hardware store?

New flexible air duct

my only suggestion is try not to use flexible duct,.... instead... use the solid ducting material... to make your job more solid/robust rather subject to deteriation... and you certainly need a solid adaptor at the inteface you are talking about which is the most difficult part... then route out from there using solid piping isn't that difficult.... you may be able to build the interface yourself with the supplies from HD....